System and method for an AC powered downhole gauge with capacitive coupling

ABSTRACT

Borehole instruments are powered by AC instrument power transmitted on all three phases of a power cable concurrently carrying three phase motor power, the instrument power transmitted at a multiple of the motor power frequency and having a corresponding fraction of the motor power voltage, and received via a capacitive coupling sufficient to withstand high-voltage cable insulation testing. The phase-to-neutral motor power provides approximately the same power level to the borehole instruments if a phase shorts to ground.

BACKGROUND

1. Field of Invention

The present invention is directed, in general, to borehole productionpower systems and, more specifically, to powering downhole measurementand control mechanisms using alternating current power over the threephase power cable.

2. Description of Prior Art

Downhole electrical submersible pump (ESP) instrumentation systems aretypically powered by impressing a direct current (DC) voltage relativeto ground on all three phases of the three phase power cable carryingpower to the pump motor. As a result, the normally floating phases ofthe three phase power system no longer float but are instead maintainedat this voltage (on average).

The impressed DC voltage is used to power measurement devices within theborehole and, in most cases, to transmit data back to the surface.However, these direct current systems limit high voltage insulationtesting of the three phase cable, and become inoperable if any phase ofthe three phase power system shorts to ground.

There is, therefore, a need in the art for an improved system forpowering downhole instrumentation over a three phase power cablecarrying power to a pump motor.

SUMMARY OF INVENTION

To address the above-discussed deficiencies of the prior art the presentinvention provides for use in a borehole production system, ACinstrument power for borehole instruments that is transmitted on allthree phases of a power cable concurrently carrying three phase motorpower, the instrument power transmitted at a multiple of the motor powerfrequency and having a corresponding fraction of the motor powervoltage. The instrument power is received via a capacitive couplingsufficient to withstand high-voltage cable insulation testing. Thephase-to-neutral motor power provides approximately the same power levelto the borehole instruments if a phase shorts to ground.

The foregoing has outlined rather broadly the features and technicaladvantages of the present invention so that those skilled in the art maybetter understand the detailed description of the invention thatfollows. Additional features and advantages of the invention will bedescribed hereinafter that form the subject of the claims of theinvention. Those skilled in the art will appreciate that they mayreadily use the conception and the specific embodiment disclosed as abasis for modifying or designing other structures for carrying out thesame purposes of the present invention. Those skilled in the art willalso realize that such equivalent constructions do not depart from thespirit and scope of the invention in its broadest form.

BRIEF DESCRIPTION OF DRAWINGS

Some of the features and benefits of the present invention having beenstated, others will become apparent as the description proceeds whentaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a borehole production system including a boreholemeasurement/control device and pump motor power system according to oneembodiment of the present invention;

FIG. 2 depicts in greater detail the gauge subassembly for use within aborehole production system according to one embodiment of the presentinvention; and

FIG. 3 graphically illustrates the relationship between instrument andmotor power on a three phase cable within a borehole production systemaccording to one embodiment of the present invention.

While the invention will be described in connection with the preferredembodiments, it will be understood that it is not intended to limit theinvention to that embodiment. On the contrary, it is intended to coverall alternatives, modifications, and equivalents, as may be includedwithin the spirit and scope of the invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF INVENTION

The present invention will now be described more fully hereinafter withreference to the accompanying drawings in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theillustrated embodiments set forth herein; rather, these embodiments areprovided so that this disclosure will be through and complete, and willfully convey the scope of the invention to those skilled in the art.Like numbers refer to like elements throughout.

FIG. 1 depicts a borehole production system including a boreholemeasurement/control device and pump motor power system according to oneembodiment of the present invention. Borehole production system 100 inthe exemplary embodiment includes an electrical submersible pump andmotor subassembly 101 lowered within a borehole 102 by means ofproduction tubing 103.

The motor portion of pump and motor subassembly 101 is powered by threephase power transmitted from the surface over a three phase cable 104. Apower source 105 provides the three phase power transmitted to themotor. Those skilled in the art will recognize that power source 105 maycomprise a variety of components, such as, for example, a generator orconnection to a power distribution grid, a converter and/or inverter,and/or a variable frequency drive (VFD) or variable speed drive (VSD).

Power source 105 in the present invention also includes a subassembly106 for generating single phase alternating current (AC) power to betransmitted over three phase cable 104 for use in powering measurementand/or control devices within a gauge subassembly 107 located within theborehole 102. The single phase power for the measurement and/or controldevices differs in frequency and peak amplitude from the three phasepower for the pump motor, as described in further detail below.

Borehole production system 100 includes at least one gauge subassembly107 disposed within the borehole. Gauge subassembly 107, described infurther detail below, measures such characteristics as temperature,pressure, cut, flow rate, or other parameters of the fluid being pumped.Alternatively, gauge subassembly 107 may measure intake pressure,temperature or flow rate, outlet pressure, temperature or flow rate,revolutions per minute, or other operating parameters of the motor/pumpsubassembly 101. In still further embodiments, gauge subassembly 107 mayinclude, with or without any measurement devices, control mechanisms foropening or closing valves or for operating other electrical ormechanical devices.

While only one gauge subassembly 107 is depicted in FIG. 1, thoseskilled in the art will recognize that any number of such subassembliesmay be employed, and at various locations along the borehole 102 such asbelow the motor, between the motor and pump, above the pump and/or atthe seal, proximate to a packer, at the wellhead (for a subsea well),etc.

In the exemplary embodiment, gauge subassembly 107 is connected to theneutral point termination of a Y-connected three phase motor within themotor/pump subassembly 101. Alternatively, however, the gaugesubassembly (or each gauge subassembly, in the case of a boreholeproduction system including multiple such subassemblies) may beconnected to one or more conductors within three phase power cable 104,at any location along borehole 102.

FIG. 2 depicts in greater detail the gauge subassembly for use within aborehole production system according to one embodiment of the presentinvention. Gauge subassembly 107 includes a capacitor 200 connected tothe motor neutral point or to another appropriately formed Y-point(formed from inductors, capacitors or both) coupled to the three phasepower cable 104. Capacitor 200 is appropriately sized to transferinstrument power from the three phase power cable 104, and is rated at asufficiently high voltage, to allow high voltage testing (megging) ofthe insulation for the three phase power cable 104 and other downholeequipment.

Capacitor 200 is coupled by a transformer 201 to a measurement and/orcontrol device 202. The power passed through capacitor 200 andtransformer 201 is employed to drive electrical components within themeasurement and/or control device 202.

FIG. 3 graphically illustrates the relationship between instrument andmotor power on a three phase cable within a borehole production systemaccording to one embodiment of the present invention. The signalsdepicted illustrate power impressed on the three phase power cable 104.In an exemplary embodiment, the individual conductors of three phasepower cable 104 are each driven by one phase A, B or C of the threephase power signal 300 (collectively signals 300A, 300B and 300C). Allthree phases are also driven by an AC voltage 301 with respect toground, which is employed as the instrument power formeasurement/control device(s) 202 within gauge subassembly 107. Althoughshown without any direct current offset in the exemplary embodiment, thepower on the three phase power cable 104 may also have aground-referenced direct current component, in which case theinstrumentation voltage 301 would be offset from neutral.

The instrument power 301 is set at a voltage that is on the order ofone-tenth (a factor of 10 less than) the voltage of the motor power 300transmitted on the three phase power cable 104, and with a frequencythat is on the order of ten times higher than the frequency of the motorpower 300 (illustrated in FIG. 3 by the period of the half cycles forthe respective signals 300 and 301). For example, instrument power 301may be approximately 80-100 volts (V) transmitted at 600 Hertz (Hz)while motor power is 1,000 to 2,500 V (peak, up to 4,160 voltsphase-to-phase) transmitted at 60 Hz.

As described above, the gauge subassembly is preferably connected to aY-point coupled to three phase power cable 104. At such a Y-point in abalanced system (with no phase shorted to ground), only the ACinstrument power 301 appears at the Y-point, together with any DCoffset. When a phase shorts to ground, however, the cumulative downholemotor phase-to-neutral power for the remaining two phases (or singlephase, if two phases short to ground) appears at the Y-point. In such anevent, the downhole instrumentation 202 is able to use the motor powervoltage for power in lieu of the normal instrument power 301. The highervoltage but reduced frequency of the downhole motor power couples intothe instrumentation 202 at relatively the same power level, since theimpedance of capacitive coupling 200 is inversely proportional tofrequency and the frequencies and voltages of motor power 300 andinstrument power 301 were selected to be inversely related as describedabove. Such a system may also be used where the voltage of motor power300 is much lower or higher than the exemplary numbers listed above bysimply adjusting the frequency and voltage of the instrument power 301with which gauge 107 is designed to operate.

In one embodiment of the present invention, the inductance oftransformer 201 may be selected to resonate with the capacitor 200 atthe frequency of the instrument power 301, providing a low impedancepath for the instrument power and a higher impedance path for the motorpower (when a phase is grounded and the motor power appears at theY-point).

The above-described embodiments disregard data transfer over three phasepower cable 104. However, one or more signaling units 110 may beseparately coupled to power cable 104 in accordance with the known artto provide modulation of a DC bias or offset on power cable 104 or bysuperimposition of a radio frequency (RF) signal in accordance with theknown art. Such signaling unit(s) 110 may be communicably coupled togauge assembly 107 separately from the common connection to power cable104, providing instrumentation 202 with alternate paths for receivingpower and signaling.

An alternative embodiment of the present invention when the AC powersystem is floating involves the transfer of data from the surfacedownhole, from the borehole to the surface, or both by modulation of theAC current demand by a controller (not shown) in the downholeinstrumentation 202. The downhole information may thereby be transmittedto the surface as a linear load change proportional to the AC signal, oras discrete levels transferring encoded data. When encoded data istransferred, the AC coupling system allows a much higher data rate thanDC powered data signaling systems since the coupling filter timeconstants are much shorter. Using two or more discrete levels willfurther enhance the data rate since each data sample will be shorter.Data may be transferred from a controller 108 at the surface to downholeinstrumentation 202 (concurrently with, i.e. bi-directionally, or duringseparate time periods for data transfer from the borehole to thesurface) by shifting the frequency of instrument power 301 by apredetermined deviation, or by briefly interrupting the power signal301. These techniques allow data to be transferred using the ACinstrument power signal 301 as long as the power system is floating.

The present invention provides power from the surface to downholeinstrumentation within a borehole over a three phase power cable, eitherseparately from or concurrently with the three phase power for adownhole motor, in a manner tolerating shorting of a conductor withinthe three phase power cable to ground and/or high voltage insulationtesting.

It is important to note that while embodiments of the present inventionhave been described in the context of a fully functional system andmethod embodying the invention, those skilled in the art will appreciatethat the mechanism of the present invention and/or aspects thereof arecapable of being distributed in the form of a computer readable mediumof instructions in a variety of forms for execution on a processor,processors, or the like, and that the present invention applies equallyregardless of the particular type of signal bearing media used toactually carry out the distribution. Examples of computer readable mediainclude but are not limited to: nonvolatile, hard-coded type media suchas read only memories (ROMs), CD-ROMs, and DVD-ROMs, or erasable,electrically programmable read only memories (EEPROMs), recordable typemedia such as floppy disks, hard disk drives, CD-R/RWs, DVD-RAMs,DVD-R/RWs, DVD+R/RWs, flash drives, and other newer types of memories,and transmission type media such as digital and analog communicationlinks. For example, such media can include both operating instructionsand/or instructions related to the system and the method steps describedabove.

It is to be understood that the invention is not limited to the exactdetails of construction, operation, exact materials, or embodimentsshown and described, as modifications and equivalents will be apparentto one skilled in the art. In the drawings and specification, there havebeen disclosed illustrative embodiments of the invention and, althoughspecific terms are employed, they are used in a generic and descriptivesense only and not for the purpose of limitation. Accordingly, theinvention is therefore to be limited only by the scope of the appendedclaims.

What is claimed is:
 1. A method of powering a motor and an instrument ofa submersible pump assembly, comprising: (a) providing a three conductorpower cable from a surface location to the motor, which has a Y-point ona lower end of the motor; (b) electrically coupling a capacitor to theY-point and a side of the capacitor opposite the Y-point to theinstrument; (c) producing three phase motor power for the motor andsimultaneously producing single phase instrument power on the threeconductor power cable, the instrument power being at a higher frequencyand lower voltage than the motor power, the instrument power voltageexisting at the Y-point and passing through the capacitor to theinstrument, the motor power having a motor voltage that is zero at theY-point; (d) in the event one of the conductors of the power cableshorts to ground, which causes the motor voltage to occur at theY-point, an impedance of the capacitor increases due to the lowerfrequency of the motor power than the instrument power, thereby passingto the instrument a reduced level of motor voltage; and (e)bi-directionally transmitting data on the three conductor power cablebetween the surface location and the instrument by modulating theinstrument power, interrupting a frequency of the instrument power, orshifting a frequency of the instrument power.
 2. The method according toclaim 1, further comprising prior to step (c), applying between groundand each conductor of the three conductor power cable a DC test voltage,which is blocked from the instrument by the capacitor.
 3. The methodaccording to claim 1, wherein step (b) further comprises: coupling theinstrument to the capacitor via a transformer, the transformer and thecapacitor being selected to provide a low impedance path for theinstrument power and a high impedance path for the motor power.
 4. Themethod according to claim 1, wherein the instrument comprises: at leastone gauge subassembly for measuring an operating characteristic orcontrolling a device; and step (c) further comprises: communicativelycoupling a signaling unit to the at least one gauge subassembly fortransmitting data on the three conductor power cable.
 5. The method ofclaim 1, wherein step (c) comprises: providing the instrument power witha voltage that is approximately a fraction 1/N of the motor powervoltage and a frequency that is approximately a multiple N of the motorpower frequency.
 6. An electrical submersible pump system comprising: athree conductor power cable extending from a surface location into aconduit; an electrical submersible pump motor within the conduit andcoupled to the three conductor power cable, the motor having windings ata lower end joined to form a Y-point; a capacitor and transformercoupled to the Y-point; an instrument coupled to the transformer, whichis connected through the capacitor to the Y-point; a power source at thesurface coupled to the three conductor power cable, the power sourceproducing three phase motor power to operate the motor and producingsingle phase instrument power for simultaneous transmission on theconductors of power cable, wherein the instrument power has a voltagethat is approximately a fraction 1/N of the motor power voltage and afrequency that is approximately a multiple N of the motor powerfrequency; a controller within the instrument modulating AC currentdrawn by the instrument from the instrument power to transmit data onthe three conductor power cable to the surface location; and one or moresignaling units separately coupled to the three conductor power cableand transmitting data over the three conductor power cable to theinstrument either by temporarily shifting a frequency of the instrumentpower or by temporarily interrupting tin the instrument power; and thecapacitor being sized so that when the Y-point has a neutral motor powervoltage and the instrument power is transmitted over the three conductorcable, the capacitor passes the instrument power at the instrument powerfrequency to operate the instrument, and so that when a conductor of thethree conductor power cable shorts to ground, creating motor powervoltage at the Y-point, the impedance of the capacitor increases to passa fraction of the motor power voltage at the motor power frequency tooperate the instrument.
 7. The system according to claim 6, wherein thecapacitor blocks from the instrument high DC test voltage during testingof the three conductor power cable insulation.